The present invention relates to a tunable optical device, and more particularly to an optical filter including at least two arrays of reflective elements, such as Bragg gratings, wherein the tunable spectral range of the optical filter is greater than the tunable spectral range of the reflective elements.
The telecommunication industry is turning towards optical networks to provide higher capacity (bandwidth) of data transfer for new applications. Consequently, the industry is continually developing technology to increase the bandwidth of the optical networks, which includes the technology of wavelength division multiplexing (WDM). A number of optical signals can be carried simultaneously on the optical fiber by modulating each signal on a different wavelength of light. The light propagating through a single optical fiber therefore includes a plurality of wavelength bands, referred to as channels, wherein each channel or wavelength band is centered at a reflection wavelength. The wavelength bands are sufficiently separated so that they do not overlap. Typically, networks that carry about eight channels or more are referred to as dense wavelength division multiplexing (DWDM) systems, such systems can carry greater than 200 channels.
The technology of optical networks is heading toward a more dynamic, flexible, and intelligent networking architecture to improve service delivery time. A key element of the emerging optical network is a tunable optical filter for selectively filtering a channel from the DWDM light propagating through the optical network. The tunable channel filter enables a channel to be dynamically switched, routed, monitored and conditioned.
It is known in the art of optical networks that Bragg gratings written in an optical fiber may be used to act as a tunable filter, as is described in U.S. Pat. No. 5,815,299, entitled xe2x80x9cMethod and System for Equalizing Respective Power Levels of Channels of a Received Optical Frequency Division Multiplexed Signalxe2x80x9d to Bayart et al.; U.S. Pat. No. 5,995,255, entitled xe2x80x9cConcatenated Fiber Grating Optical Monitorxe2x80x9d to Giles, and U.S. Pat. No. 5,579,143, entitled xe2x80x9cOptical System With Tunable In-Fiber Gratingsxe2x80x9d to Huber.
The tunable filter described in the aforementioned U.S. Pat. Nos. 5,815,299 and 5,995,155 use a single fiber Bragg grating to filter a selected optical channel or a limited number of channels from broadband light tapped off an optical network. Consequently, the optical system requires a number of tunable filters to monitor or condition each optical channel. In some instances, the number of tunable filters can equal the number of optical channels.
It is also known in the art to add a light signal to a fiber optic transmission line using a device comprising a pair of substantially identical Bragg-Evanescent-Couplers (BEC) devices that are cascaded in series, whereby both BECs reflect light having wavelengths in the region of xcexn, as described in U.S. Pat. No. 5,459,801 to Snitzer, entitled xe2x80x9cCoupler Used To Fabricate Add-Drop Devices, Dispersion Compensators, Amplifiers, Oscillators, Superluminescent Devices, and Communications Systemsxe2x80x9d.
It is further known, as described in U.S. Pat. No. 5,841,918 to Li, entitled xe2x80x9cWavelength and Bandwidth Tunable Optical Systemxe2x80x9d of a system for modifying an input optical signal by reducing its bandwidth and/or modifying its central wavelength. The system includes at least two optical filters optically coupled to a circulator. The first filter produces a predetermined reflected band of the input optical signal. The second filter produces a predetermined transmission band as part of the reflected band. One or both of the filters are tunable to modify the wavelength and/or bandwidth of the optical filter.
An object of the present invention is to provide a tunable optical filter having a pair of grating elements, which are tunable over a narrow range, that enable the effective optical filtering characteristics of the optical filter to be tuned over a spectral band of the broadband light, wider than that of the grating elements.
In accordance with an embodiment of the present invention, a tunable optical filter device includes a first optical filter having a plurality of first reflective elements. Each of the first reflective elements has a respective first reflective filter function centered at respective reflection wavelengths for reflecting a respective first wavelength band of light. Each of the first reflective filter functions is spaced and substantially non-overlapping. A second optical filter is optically connected to the first optical filter to receive the first wavelength bands of light. The second optical filter includes a plurality of second reflective elements. Each of the second reflective elements has a respective second reflective filter function centered at respective reflection wavelengths for reflecting a respective second wavelength band of light. Each of the second reflective filter functions is spaced and substantially non-overlapping. One of the first and second optical filters is tunable to overlap at least one of the first reflective filter functions and one of the second reflective filter functions.
In accordance with another embodiment of the present invention, a method for selectively filtering an optical wavelength band from an input light includes providing a first optical filter including a plurality of first reflective elements. Each of the first reflective elements has a respective first reflective filter function centered at respective reflection wavelengths for reflecting a respective first wavelength band of light. Each of the first reflective filter functions being spaced and substantially non-overlapping. The method further includes directing the input light to the first optical filter, and providing a second optical filter including a plurality of second reflective elements. Each of the second reflective elements has a respective second reflective filter function centered at respective reflection wavelengths for reflecting a respective second wavelength band of light. Each of the second reflective filter functions is spaced and substantially non-overlapping. The method also includes directing the plurality of the reflected first wavelength bands to the second optical filter, and tuning one of the first and second optical filters to overlap at least one of the first reflective filter functions and one of the second reflective filter functions.
In accordance with another embodiment of the present invention, a tunable optical filter device includes a first optical filter having a plurality of reflective elements. Each of the reflective elements has a respective reflective filter function centered at respective reflection wavelengths for reflecting a respective first wavelength band of light. Each of the first reflective filter functions is spaced and substantially non-overlapping. A second optical filter is optically connected to the first optical filter to receive the first wavelength bands of light. The second optical filter includes a plurality of transmissive elements. Each of the transmissive elements has a respective transmissive filter function centered at respective transmissive for transmitting a respective second wavelength band of light. Each of the transmissive filter functions is spaced and substantially non-overlapping. One of the first and second optical filters is tunable to overlap at least one of the reflective filter functions and one of the transmissive filter functions.
In accordance with another embodiment of the present invention, a tunable optical filter device includes a first optical filter having a plurality of transmissive elements. Each of the transmissive elements has a respective transmissive filter function centered at respective transmissive wavelengths for transmitting a respective first wavelength band of light. Each of the transmissive filter functions is spaced and substantially non-overlapping. A second optical filter is optically connected to the first optical filter to receive the first wavelength bands of light. The second optical filter includes a plurality of transmissive elements. Each of the reflective elements has a respective reflective filter function centered at respective reflection wavelengths for reflecting a respective wavelength band of light. Each of the reflective filter functions is spaced and substantially non-overlapping. One of the first and second optical filters is tunable to overlap at least one of the transmissive filter functions and one of the reflective filter functions.
In accordance with another embodiment of the present invention, a tunable optical filter device includes a first optical filter having a plurality of first reflective elements. Each of the first reflective elements has a respective first reflective filter function centered at respective reflection wavelengths for reflecting a respective first wavelength band of light. Each of the first reflective filter functions is spaced and substantially non-overlapping. A second optical filter is optically connected to the first optical filter to receive the first wavelength bands of light. The second optical filter includes a second reflective element. The second reflective element has a respective second reflective filter function centered at respective reflection wavelength for reflecting a respective second wavelength band of light. One of the first and second optical filters is tunable to overlap only one of the first reflective filter functions and the second reflective filter function.
In accordance with another embodiment of the present invention, a tunable optical filter device includes a first optical waveguide having a plurality of first gratings. Each of the first gratings has a respective first reflective filter function centered at respective reflection wavelengths for reflecting a respective first wavelength band of light. Each of the first reflective filter functions is spaced and substantially non-overlapping. A second optical waveguide is optically connected to the first optical waveguide to receive the first wavelength bands of light. The second optical waveguide includes a plurality of second gratings. Each of the second gratings has a respective second reflective filter function centered at respective reflection wavelengths for reflecting a respective second wavelength band of light. Each of the second reflective filter functions is spaced and substantially non-overlapping. One of the first and second optical waveguides is tunable to overlap only one of the first reflective filter functions and one of the second reflective filter functions.